This study will explore the plasticity of developing cerebral cortex during the postnatal period, using the pyramidal motor system of the rat as a model. The ability of sensorimotor cortex to reorganize itself after perinatal injury and possible effects of motor activity on such reorganization will be studied. Observations will be made in 4 groups of animals. The first is a group of immature (age 1-2 week) and mature (age 90 days) normal rats, reared under standard conditions. Pyramidal tract neurons in cerebral cortex will be labelled by retrograde transport of horseradish peroxidase. The tracer will be injected unilaterally into the cervical enlargement of spinal cord. The distribution and number of labelled cortical neurons will be determined. This group will serve as a control for the experimental groups. One experimental group will have restraint of one forelimb during the period of plasticity of the pyramidal motor system. Another group will have a hemispherectomy during the first postnatal week. A third experimental group will have both a neonatal hemispherectomy and unilateral limb restraint. The limb restraint will be ipsilateral to the hemispherectomy in half, contralateral in the other half. All experimental animals will be allowed to grow to maturity (age 90 days). The number and distribution of pyramidal tract neurons in cerebral cortex will then be studied after injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the cervical enlargement of the spinal cord, both ipsilateral and contralateral to the experimental manipulations. Quantitative data will be obtained by computer-assisted methods and results in the various groups will be examined for statistically significant differences. Knowledge concerning the extent and mechanism of plasticity in postnatal cerebral cortex is of great importance for our understanding of development of cortical function, both in the normal situation and after brain injury. It is hoped that such knowledge will eventually lead to strategies for maximizing developmental potential in man, both in the normal human infant and in infants with focal brain damage. For example, a finding of increased formation of uncrossed connections related to restraint of the limb ipsilateral to motor cortex injury would suggest a possible analogous strategy for rehabilitation of infants with the hemiplegic form of cerebral palsy.